I have a spinal fusion four years ago and have been having a lot of pain in my back and muscle on my left side goin under my arm and around my side?

Need exam. It is hard to pinpoint what is causing your symptoms in this type of forum. The best thing to do would be to be in contact with your surgeon and let him/her exam you and decide what the next steps would be.
See your surgeon. You should follow up with your spine surgeon to make sure there are no issues with the hardware. If not they can recommend further imaging, meds, or physical therapy as indicated.